The present invention relates to a method and apparatus for controlling a satellite. More specifically, the method and apparatus relates to controlling the position and varying the orbit of a satellite which may be stabilized in three major axes. The present method and system is especially adapted for a geostationary satellite equipped with propelling systems and at least one position control system supported by a spin or momentum wheel.
In order to maximize the payload, especially in geostationary satellites, it is customary to propel the satellite first into an elliptical transfer orbit. Such orbit has an apogee with a height corresponding approximately to the subsequent orbital height of a geostationary satellite. Such transfer orbit has a low perigee in the range of a few one hundred kilometers. Following any desired number of orbits in the transfer orbit, the satellite is lifted in the apogee of the transfer orbit into an orbit in synchronism with the earth, whereby the lifting is accomplished with a driving or propelling system also referred to as the apogee engine. In addition to the apogee engine there is provided an orbit correction system which serves for precisely adjusting the orbital speed in terms of centimeters per second in order to assure a good synchronization with the rotation of the earth. These propelling or drive systems also control the steering of the satellite into the desired position and the maintaining of such position during the useful life of the satellite thereby compensating for any attraction caused by the sun, the moon and also correcting any orbit distortions caused by solar pressure.
Such propelling systems may also be employed for the steering of satellites into any desired flying trajectories not geostationary whereby such satellites may be intended for other missions.
Heretofore, it was customary to use in connection with satellites of the mentioned type different propelling and position control methods for the two basically different maneuver phases, namely transfer trajectory and orbit, for example, such as a geostationary orbit.
The necessity to use different position control methods during the two maneuvering phases results from the fact that heretofore a solid fuel engine was used for the apogee engine or motor. However, the thrust of solid fuel engines is relatively high. As a result, the passive gyro stabilization by means of a spin wheel or momentum wheel alone is not strong enough. This is so during the change of orbit phase as well as in any other trajectory or orbit correction which may subsequently become necessary. Such momentum or spin wheels are insufficient to take up the high moments which occur as the result of the use of solid fuel engines. Thus, according to the prior art it is customary to use in the transfer phase a momentum or spin stabilization. If the satellite is of elongated construction, the spin is applied generally around the axis of the smallest main or primary moment of inertia. An elongated satellite, incidentally, is best adapted to the space available in the nose cone of a rocket. The application of a spin about the axis of the smallest moment of inertia, however, results in a rotation which is not stable so that nutations which cannot be avoided due to disturbances, must be constantly counteracted by a positive or active nutation damping.
Solid fuel engines are also employed in order to minimize or avoid the vibration and dynamic stabilization problems which occur in the fuel tank system of liquid fuel engines. In connection with the just described technical concept of the prior art to employ a solid fuel engine as an apogee engine or as a trajectory or orbit change engine and to employ different methods of stabilization, difficult maneuver sequences cannot be avoided. Further, a high material expense which diminishes the payload and which increases the costs can also not be avoided. Furthermore, it is difficult to guess that nutations that might occur in outer space, not to speak of testing such nutations. As a result, it is very critical to design and construct the active nutation damping. In addition, it is sensible to make the trajectory or orbit corrections, which are necessary during the entire useful life of a satellite, by means of liquid fuel drive systems or engines. Thus, at least two different engine systems must be provided for the change of orbit on the one hand and for the position control or orbit correction on the other hand. Such different drive systems based on different types of fuels require separate storage and separate conduit systems as well as separate instrumentation and so forth. An example of the just described prior art technique is described in the RCA Paper, 1975 under the title "RCA's Three Axes Communication Satellite" (RCA-SATCOM Satellite).